Detectors for detecting relative distance between objects are known. One such detector is disclosed in Japanese Kokai Patent Application No. 2001-084497. With this position detector, the amount of movement in the vertical orientation between picture frames for an object detected on photographed images can be measured and the amount of change in the vehicle pitching angle can be calculated based on the amount of movement. However, in the existing device, the amount of pitching change in the current frame is calculated based on the amount of movement in the vertical orientation of an object between frames, which is added to the vanishing line position in the previous frame, and the vanishing line position in the current frame is calculated. There may be instances, however, where this may result in the position of the vanishing line for the current frame being miscalculated when movement of an object in the vertical orientation between frames is detected as a result of a change in the relative distance between the vehicle and the object. Accordingly, it is desirable to develop an apparatus and method for more accurately determining a position of a vanishing line used in calculating the distance between the vehicle and an object.